An air staged combustion technology is widely adopted in the technical field of low NOX combustion of coal boilers at present. Known from searching the documents related to the prior art, “Present Status of Low-NOx Combustion Technology” (Bi Yusen, Thermal Power Generation, 2000, No.2) introduced an integrated air-staged direct flow burner and coaxial combustion systems CFS I and CFS II of a tangential fired boiler developed by ABB-CE Company in detail, and on this basis, a low NOX combustion system of separated over fire air (SOFA) or close coupled over fire air (CCOFA) is added.
Oxygen-deficiency reductive combustion is implemented in a primary burner zone, the excess air coefficient is less than 1, and the remaining secondary air is supplied through overfire air, so that air staged combustion is realized and the objective of reducing the production amount of NOX is fulfilled. When the technology is used for bituminous coal and lean coal fired boilers, emission of NOX may reach 250-650 mg/Nm3. However, the technology has shortcomings. Through the CFS I and CFS II technologies, primary air jet flow and secondary air jet flow injected into the furnace center form a coaxial positive-negative double-tangential combustion mode in opposite directions or the same direction, but because the adjacent primary air jet flow and deflecting secondary air jet flow entrain each other, a part of secondary air enters a coal volatile matter separation and combustion zone to generate NOX. The primary combustion zone presents a reductive atmosphere, so that slagging and high-temperature corrosion are aggravated.
The emission of NO may be reduced by adding the height of the separated over fire air (SOFA) or increasing the amount of the separated over fire air (SOFA), but the combustion efficiency is reduced, the flue gas temperature of a hearth outlet rises, and the flue gas temperature deviation is increased. By adopting multistage corner-placed or wall-type tangential jet overfire air, the combustion efficiency is improved, and at the same time, because the air enters a “center zone” to make NO reductive matters such as HCN, CO and NHi etc. gathered in the “center zone” of the hearth be converted into NO, the reduction rate of NO in a reduction zone is reduced. By adopting such centralized tangential jet overfire air, the rising high-temperature flue gas inevitably revolves, so that air distribution and air speed in an burnout zone are non-uniform, coal ash granules are easily settled on the heating surface of the cross section of the hearth in the this zone, and a centralized high-temperature combustion zone is easily formed to cause slagging in this zone or upward movement of slagging to a platen superheater zone.